Surface sensitivity is an important factor that determines the minimum amount of biomolecules detected by surface plasmon resonance (SPR) sensors. We propose the use of oblique-angle-induced Fano resonances caused by two-mode coupling or three-mode coupling between the localized SPR mode and long-range surface plasmon polariton modes to increase the surface sensitivities of silver capped nanoslits. The results indicate that the coupled resonance between the split SPR (&minus;kSPR) and cavity modes (two-mode coupling) has a high wavelength sensitivity for small-angle incidence (2&deg;) due to its short decay length. Additionally, three-mode coupling between the split SPR (&minus;kSPR), substrate (+kSub) and cavity modes has a high intensity sensitivity for large-angle incidence due to its short decay length, large resonance slope and enhanced transmission intensity. Compared to the wavelength measurement, the intensity measurement has a lower detectable (surface) concentration below 1&thinsp;ng/ml (0.14&thinsp;pg/mm2) and is reduced by at least 3 orders of magnitude. In addition, based on the calibration curve and current system noise, a theoretical detection limit of 2.73&thinsp;pg/ml (0.38&thinsp;fg/mm2) can be achieved. Such a surface concentration is close to that of prism-based SPR with phase measurement (0.1&ndash;0.2&thinsp;fg/mm2 under a phase shift of 5 mdeg).

f7: Fabrication of capped nanoslits and optical setup for angular transmission spectrum measurement.(a) A process flowchart for the fabrication of silver capped nanoslits. (b) The SEM and optical images (inset) of the fabricated silicon template. The groove width is 80 nm and the area of each slit array is 2 mm × 2 mm. (c) The SEM and optical images (inset) of the silver capped nanoslits. (d) The optical setup for measuring transmission spectra at different angles.

Mentions:
Figure 7a shows a process flowchart for the fabrication of the sliver capped nanoslits. The nanostructures were produced on a cyclic olefin polymer (COP) substrate using hot embossing nanoimprint lithography and metal sputtering. First, periodic nanogrooves of 80 nm in width, 80 nm in depth and 520 nm (or 500 nm) in period were fabricated on a silicon substrate using electron beam lithography and a reactive ion etching method. A 300-nm-thick ZEP-520 resistor (ZEP-520, Zeon Corp, Tokyo, Japan) was spin-coated on a 525-μm-thick silicon substrate. An electron-beam writing system (ELS 7000, Elionix, Japan) was used to write groove arrays with various groove periods. The patterns were then transferred to the silicon substrate using a reactive ion etching machine (Oxford Instrument, plasmalab 80plus). Figure 7b shows the scanning electron microscope (SEM) and optical images (inset) of the fabricated silicon template. The nanostructures on the silicon were then imprinted onto a 178-μm-thick COP film using hot embossing nanoimprint equipment (EHN-3250, Engineering System Co. Ltd.) After sputtering an 80-nm-thick silver film on the imprinted plastic substrate, the silver capped nanoslit arrays were produced. Figure 7c shows the SEM and optical images (inset) of the silver capped nanoslits. There were 4 arrays on the chip. The area of each periodic nanostructure was 2 × 2 mm2.

f7: Fabrication of capped nanoslits and optical setup for angular transmission spectrum measurement.(a) A process flowchart for the fabrication of silver capped nanoslits. (b) The SEM and optical images (inset) of the fabricated silicon template. The groove width is 80 nm and the area of each slit array is 2 mm × 2 mm. (c) The SEM and optical images (inset) of the silver capped nanoslits. (d) The optical setup for measuring transmission spectra at different angles.

Mentions:
Figure 7a shows a process flowchart for the fabrication of the sliver capped nanoslits. The nanostructures were produced on a cyclic olefin polymer (COP) substrate using hot embossing nanoimprint lithography and metal sputtering. First, periodic nanogrooves of 80 nm in width, 80 nm in depth and 520 nm (or 500 nm) in period were fabricated on a silicon substrate using electron beam lithography and a reactive ion etching method. A 300-nm-thick ZEP-520 resistor (ZEP-520, Zeon Corp, Tokyo, Japan) was spin-coated on a 525-μm-thick silicon substrate. An electron-beam writing system (ELS 7000, Elionix, Japan) was used to write groove arrays with various groove periods. The patterns were then transferred to the silicon substrate using a reactive ion etching machine (Oxford Instrument, plasmalab 80plus). Figure 7b shows the scanning electron microscope (SEM) and optical images (inset) of the fabricated silicon template. The nanostructures on the silicon were then imprinted onto a 178-μm-thick COP film using hot embossing nanoimprint equipment (EHN-3250, Engineering System Co. Ltd.) After sputtering an 80-nm-thick silver film on the imprinted plastic substrate, the silver capped nanoslit arrays were produced. Figure 7c shows the SEM and optical images (inset) of the silver capped nanoslits. There were 4 arrays on the chip. The area of each periodic nanostructure was 2 × 2 mm2.

Surface sensitivity is an important factor that determines the minimum amount of biomolecules detected by surface plasmon resonance (SPR) sensors. We propose the use of oblique-angle-induced Fano resonances caused by two-mode coupling or three-mode coupling between the localized SPR mode and long-range surface plasmon polariton modes to increase the surface sensitivities of silver capped nanoslits. The results indicate that the coupled resonance between the split SPR (&minus;kSPR) and cavity modes (two-mode coupling) has a high wavelength sensitivity for small-angle incidence (2&deg;) due to its short decay length. Additionally, three-mode coupling between the split SPR (&minus;kSPR), substrate (+kSub) and cavity modes has a high intensity sensitivity for large-angle incidence due to its short decay length, large resonance slope and enhanced transmission intensity. Compared to the wavelength measurement, the intensity measurement has a lower detectable (surface) concentration below 1&thinsp;ng/ml (0.14&thinsp;pg/mm2) and is reduced by at least 3 orders of magnitude. In addition, based on the calibration curve and current system noise, a theoretical detection limit of 2.73&thinsp;pg/ml (0.38&thinsp;fg/mm2) can be achieved. Such a surface concentration is close to that of prism-based SPR with phase measurement (0.1&ndash;0.2&thinsp;fg/mm2 under a phase shift of 5 mdeg).